Power supply control circuit

ABSTRACT

A power supply control circuit for controlling power supply through a power switch to a battery-operated circuit. The control circuit includes circuitry for keeping the power supply to the battery-operated circuit for a predetermined holding time after the power switch has been turned off, the holding time being reduced as consumption of the battery develops.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a power supply control device including circuitry for holding the power supply from a battery to a battery-operated circuit for a certain time period after a power switch is turned off.

2. Description of the Prior Art

Conventionally, a camera exposure meter circuit, automatic exposure control circuit or the like is provided with circuitry for supplying power switch on in response to the shutter buttom operation at its half-depression position, which is shallower than the stroke where the shutter release takes place and for maintaining or holding the power supply operative to supply power for a certain time period even if an operator releases his finger from the shutter button to facilitate camera handling.

In a camera with such circuitry, an operator can view the exposing condition or exposure control status for a certain time period after releasing his finger from the shutter button. In such conventional power supply holding circuits, however, the power supply holding time is set as a constant value even where a battery is weak or depleted. This kind of circuit with a power supply holding function, which is directed only to the purpose of facilitating camera handling, and is not necessarily required to take a photograph, has a disadvantage in that consumption of a batter is hastened.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide an improved power supply control circuit in which the power supply holding time is reduced as the voltage of a power source decreases to prevent the battery from being wasted.

The foregoing object and other objects are accomplished according to the present invention in a power supply control circuit for controlling power supply to a battery-operated circuit, the control circuit comprising a circuit for starting time-measuring upon a manual operation and producing a time-measuring signal for a predetermined time period from the starting time, a circuit for holding the power supply operative to supply power while the time-measuring signal is being produced, and a circuit for detecting the battery voltage and reducing the predetermined time period to be time-measured in the time-measuring circuit as the battery voltage goes down.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a circuit diagram of a first embodiment of power supply control circuit according to the present invention.

FIG. 2 is a plot of power supply holding time versus battery voltage in the circuit of FIG. 1.

FIG. 3 is a circuit diagram of a second embodiment of power supply control circuit according to the present invention.

FIG. 4 is a plot of a power supply holding time versus battery voltage in the circuit of FIG. 3.

FIG. 5 is a circuit diagram of a third embodiment of power supply control circuit according to the present invention.

FIG. 6 is a plot of power supply holding time versus battery voltage in the circuit of FIG. 5.

DETAILED DESCRIPTION OF THE EMBODIMENTS First Embodiment

A first embodiment of power supply control circuit in accordance with the invention is shown in FIG. 1. Battery E supplies exposure value arithmetic circuit 1 with power upon the closure of power switch SW₁. In an exposure meter, power switch SW₁ is closed through the depression operation on a push button provided therein and opened through the release from the push button. In a camera including an exposure metering circuit, the power switch is closed in response to the half-depression operation on a shutter button and opened through the release operation from the shutter button. Resistors R₂, R₃, R₄ and R₅, capacitor C₁ and transistors Q₁ and Q₂ constitute a power supply holding circuit. Exposure arithmetic circuit 1 receives as input information a subject brightness detected through photodiode PD, film sensitivity, shutter speed and aperture value respectively determined by setting variable resistor R_(f) and evaluates them as to whether they meet a proper exposure condition. The evaluation result is displayed on light emission diodes LED₁ and LED₂ in a predetermined mode. Transistor Q₃ which serves to control the power supply to arithmetic circuit 1, is biased by means of resistors R₆ and R₇ so as to be turned off when a power supply voltage V to arithmetic circuit 1 falls to a lower limit voltage V₁ for arithmetic circuit 1. And also, resistors R₄ and R₅ are arranged so that a voltage slightly higher than the lower limit voltage V₁ becomes the critical voltage for ON or OFF state in transistor Q₂. The working of the circuit will be explained in three cases of battery consumption condition. In the explanation, V is a voltage of the battery, V₁ a lower limit voltage which assures the proper operation of arithmetic circuit 1 as a line voltage and V₂ a voltage higher than V₁ which does not fall to lower limit voltage V₁ during the power supply holding time period (in other words, V₂ is a voltage which assures that arithmetic circuit 1 can normally operate for the power supply holding time period.).

V>V₂

In this case of battery condition, battery E has a sufficient or adequate voltage. As power switch SW₁ is closed, the voltage developed on resistors R₂ and R₃ biases transistor Q₁ to its ON state. The collector current of transistor Q₁ flows through resistors R₄ and R₅ and develops a bias voltage which causes transistors Q₂ and Q₃ to be turned ON. Transistors Q₁ and Q₂ constitute a positive feedback circuit. In this embodiment, during the period while the voltage across capacitor C₁ remains small, after power switch SW₁ has been opened, the above-mentioned positive feedback operation keeps transistor Q₁ in its ON state. While power switch SW₁ remains closed, the potential at one terminal a of capacitor C₁ can not rise over the forward voltage of diode D₂ because terminal a is clamped to ground through diode D₂ and power switch SW₁. Accordingly, the bias voltage which causes transistor Q₁ to be turned ON is unchanged. The conduction path of transistor Q₃ in its ON state forms a power supply line to arithmetic circuit 1 as long as power switch SW₁ is closed. Thereafter, when power switch SW₁ is opened, the clamping for terminal a is released. The collector current of transistor Q₂ charges capacitor C₁ and the potential of terminal a gradually rises to the level which can turn transistor Q₁ off, after a predetermined time period. When transistor Q₁ is turned OFF, the bias voltage which causes transistor Q₃ to be turned on is lost and the power line to arithmetic circuit 1 is disconnected. The rising potential at terminal a is a function of time constant determined by capacitor C₁ and resistors R₂ and R₃. During a time period from the opening of power switch SW₁ to the time when the potential at terminal a rises to the level which turns transistor Q₁ off, transistors Q₁ and Q₃ remain ON and continue supplying arithmetic circuit 1 with power to work an exposure meter. The time required to charge capacitor C₁ determines the power supply holding time period. In this embodiment, holding time measurement starts with the opening of power switch SW₁. Alternatively, it is possible to constitute a circuit in which the time measurement starts with the closure of switch SW₁.

V₂ >V>V₁

In this case of battery condition, the voltage V of battery E is nearly equal to the lower limit voltage V₁. The closure of power switch SW₁ turns transistor Q₁ on. With transistor Q₁ in its ON state, the collector current of transistor Q₁ which flows through resistors R₄ -R₇ develops bias voltages for transistors Q₂ and Q₃. As battery voltage V is higher than V₁ but lower than V₂, transistor Q₃ is turned on and supplies power to arithmetic circuit 1, while transistor Q₂ can not be turned on. While transistor Q₂ is in its OFF state, there is no potential difference between terminals a and b of capacitor C₁ because both terminals are clamped through power switch SW₁ to ground (through diodes D₂ and D₃, respectively). That is, capacitor C₁ is not charged in a reverse polarity through discharging resistor R₁ described later. Thus, polarized capacitors like tantalum electrolytic capaccitors and aluminum electrolytic capacitors are available for capacitor C₁. Now, power switch SW₁ is opened. At this time, capacitor C₁ is not charged through transistor Q₂ because transistor Q₂ is in its OFF state. The potential at terminal a immediately rises to the level which turns transistor Q₁ off and in turn transistor Q₃ off to cut off the power supply to arithmetic circuit 1. Accordingly, a user notices through the off-light state of display elements LED₁ and LED₂ in arithmetic circuit 1 that the exposure meter has stopped its operation just after power switch SW₁ was opened and is thus informed that the battery has been consumed to the condition where the battery voltage V will soon fall to lower limit voltage V₁. Even in this situation, if the operator closes power switch SW₁, the exposure metering circuit will be able to operate.

V₁ >V

When power switch SW₁ is closed, transistors Q₂ and Q₃ can not be turned ON, though transistor Q₁ can be ON. That is, the power to arithmetic circuit 1 can not be supplied. At this status, display elements LED₁ and LED₂ remain unlighted and the operator learns that battery E has been consumed or exhausted to an un-usable condition. Naturally, even immediately after power switch SW₁ has been opened, transistor Q₃ is not turned on.

The operating characteristic of the circuit of FIG. 1 is shown in FIG. 2. The variable on the vertical axis is a power supply holding time t time-measured from the opening of power switch SW₁ and the variable on the horizontal axis a battery voltage V. When V>V₂, holding time t is continuously reduced as the battery voltage goes down. This is because the charging current through resistor R₂ which biases transistor Q₁ becomes small more rapidly in a lower battery voltage.

Second Embodiment

A second embodiment of power supply control circuit in accordance with the invention is presented in FIG. 3. In this embodiment, as compared with the circuit of FIG. 1, the change of holding time t when battery voltage V is higher than V₂ is so emphasized that a user may learn the change of battery voltage in the range higher than V₂ from the change of holding time. This embodiment is designed as an exposure control circuit in a magnetic release system. For clarity, those elements in this embodiment which are functionally the same as those of FIG. 1 are identified by the same reference characters. Power switch SW₁ and switch SW₂ operate serially. As a shutter button is depressed in a half stroke, power switch SW₁ is closed. And further depression on the shutter button brings the closure of switch SW₂ which causes the magnetic release mechanism to work for taking photograph. The operation of the second embodiment is as follows.

When battery E has a sufficient or adequate voltage, the closure of power switch SW₁, which is brought by the half-depression on the shutter button, turns on transistor Q₁₀ through reverse current blocking diode D₁₀. Through the conduction path of transistor Q₁₀ in its ON state, auto-exposure control circuit 2 is supplied with power. Auto-exposure control circuit 2 receives as input information a subject brightness detected by photodiode PD, film sensitivity value and aperture value respectively determined by setting variable resistor R_(f) and arithmetically evaluate them to display a proper shutter speed value on meter M and also, when the magnetic release mechanism is operated, to control a shutter (not shown) at the proper shutter speed through shutter control magnet Mg₂. The line voltage is divided in serially connected resistors R₁₀, R₁₁ and R₁₂. An output voltage V_(A1) of voltage follow amplifier A₁ is represented as ##EQU1## where K₁ is constant. The closure of power switch SW₁ also turns transistor Q₁₁ on. With transistor Q₁₁ in its ON state, capacitor C₁₀ is shorted and thus not charged. Voltage source ES produces a constant voltage like a zener diode or band-gap regulator which works when transistor Q₁₀ is in its ON state. Current source I₁ is a current sink source which provides charge current i₁ to capacitor C₁₀ when transistor Q₁₁ is in its OFF state, as described later. When transistor Q₁₁ is turned on through the closure of switch SW₁, the voltage at non-inverting input+ of comparator A₂ is made equal to the output voltage V_(A1) of follower amplifier A₁, that is V.K. In this context, the sufficient battery voltage means that the output voltage V_(A1) =V.K₁ of follower amplifier A₁ is higher than the voltage V_(ES) of constant voltage sourse ES. Thus, comparator A₂ produces its high level output which causes transistor Q₁₂ to be turned on. As a result, the circuit has a positive feedback function which allows transistor Q₁₀ to remain in its ON state even after power switch SW₁ has been opened. A voltage V_(A3in) which is derived by dividing the line voltage V through resistors R₁₀, R₁₁ and R₁₂ is applied to non-inverting input+ of comparator A₃. The voltage V_(A3in) is represented as ##EQU2## where K₂ is constant. It will be easily understood that K₂ >K₁. Thus, the input voltages of comparators A₁ and A₃ are in a relation of V_(A3in) >V_(A1).

When the battery possesses a voltage sufficient to exhibit the relation of V.K₂ >V_(ES), comparator A₃ produces a high level which causes transistor Q₁₃ to be turned on. Through the conduction path of transistor Q₁₃ in its ON state, shutter speed meter M can indicate a controlled shutter speed value. At this state, further depression on the shutter button closes switch SW₂. The high level output of comparator A₃ turns transistor Q₁₄ on. The charge in capacitor C₁₁ which has been accumulated through a resistor by the line voltage discharges as a current for magnetizing magnet Mg₁ to actuate a magnet release mechanism (not shown) which conducts a photograph taking sequence. Once the photograph taking sequence starts, transistors Q₁₀ and Q₁₁ are adapted to keep their ON state through line 3. That is, even if power switch SW₁ is opened during the photograph taking sequence, the same condition as when the power switch is closed is secured until the completion of exposure. Thereafter, when power switch SW₁ is opened, while transistor Q₁₁ is turned off, transistor Q₁₀ continues its ON state because of the positive feedback function through transistor Q₁₂ in its ON state. As transistor Q₁₁ is turned off, capacitor C₁₀ which has so far been shorted by transistor Q₁₁ in ON state is charged by constant current sink sourse I₁. When a voltage at non-inverting input+ of comparator A₂ drops below the voltage V_(ES) of constant voltage source E_(S) which is applied to inverting input-of comparator A₂, the output of comparator A₂ becomes a low level which places transistor Q₁₂ in its OFF state. As a result, the positive feedback function is lost and then transistor Q₁₀ is turned off so that the power supply line to exposure control circuit 2 is broken. That is, the power supply holding time is determined by the charging time in capacitor C₁₀. The interval t from the opening of power switch SW₁ to the time when transistor Q₁₀ is turned off has the following relation with voltage V of battery E. ##EQU3##

Where battery voltage V goes down to the level in which V.K₁ ≦V_(ES) (that is V≦V_(ES) /K₁), power supply holding time t becomes zero.

Where V_(ES) /K₂ <V<V_(ES) /K₁, the holding time t is zero, while the power supply to exposure control circuit 2 can be conducted as long as power switch SW₁ is closed because the output voltage of comparator A₃ is a high level in the relation of V.K₂ >V_(ES). Accordingly, the user can learn that the battery is still usable but will be soon consumed to an unusable level.

Where V<V_(ES) /V₂, the output voltage of comparator A₃ is a low level which places transistor Q₁₃ in its OFF state and thus meter M does not work. The user knows from the status of meter M that battery E has been exhausted to an unusable condition. As another aspect, even if the user erroneously depresses the shutter button to a position where shutter release takes place and turns switch SW₂ on, magnet Mg₁ does not start the photograph taking sequence of the camera because transistor Q₁₄ is in its OFF state.

FIG. 4 shows a solid line the relation between voltage V of battery E and power supply holding time t during which transistor Q₁₀ maintains its ON state after power switch SW₁ is opened. V₁₂ is a voltage at a point of V=V_(ES) /K₁ where the holding time is zero. V₁₁ is a voltage at a point of V=V_(ES) /K₂ where meter M does not work upon the closure of power switch SW₁ because the output of comparator A₃ is a low level, and the photographing sequence does not start even if switch SW₂ is closed. With the resistance value of resistor R₁₁ reduced to zero so that K₁ =K₂, it is possible to make the release function inoperative at a point where the holding time is zero. The characteristic in such an operation is shown as a dash line in FIG. 4.

Third Embodiment

FIG. 5 shows a third embodiment in which the holding time control is processed in a digital manner to obtain a holding time which changes in a step manner as the battery voltage varies. Those elements in this embodiment which are functionally the same as those of FIG. 3 are identified by the same reference characters. Digital aritmetic circuit 20 analog-digital converts the brightness information of a shooting-subject detected through photodiode PD and conducts a digital calculation for proper exposure by processing the converted digital brightness information, film speed, aperture value and other exposure parameters set by digital setting circuit 21. The calculation result is transferred to digital display circuit 22. When transistor Q₁₃ is in its ON state, the calculation result is displayed in a predetermined mode on digital display 23. Digital arithmetic circuit 20 includes an oscillator circuit of a predetermined frequency. This oscillator circuit generates a clock pulse sequence required to conduct digital arithmetic calculation in circuit 20 and also provides, through clock line 6, display circuit 22 and power supply holding timer 24 with another clock pulse sequence synchronized with the former clock pulse sequence. Timer 24 can be constructed by conventional digital counter technology. When transistor Q₁₀ is placed in its ON state, timer 24 receives the clock pulses forwarded from the oscillator circuit in digital arithmetic circuit 20. At the time when power switch SW₁ is turned from its closed condition into an open condition, timer 24 starts the count and provides gates G₁ and G₂ with timing signals each of which has a transition from a high level to a low level at a different time from each other. The working is as follows.

When battery E has a sufficient voltage, the closure of power switch SW₁ actuates the positive feedback circuit consisting of transistors Q₁₀ and Q₂₀ which effect a self-holding function so that the arithmetic circuit is supplied with power. Herein, the sufficient voltage means that anyone of three step-like voltages V_(A10), V_(A11) and V_(A12) applied to non-inverting input+ of comparators A₁₀, A₁₁ and A₁₂ is higher than reference voltage V_(ES). In this situation, all the outputs of comparators A₁₀, A₁₁ and A₁₂ are in a high level. Accordingly, the output of comparator A₁₀ turns transistor Q₁₃ on. Display circuit 22 operates so that display 23 may display the control value. And also, the output of comparator A₁₀ places transistor Q₁₄ in a condition where the charge on capacitor C₁₁ is discharged through release magnet Mg₁ if release switch SW₂ is closed so that the release operation in a camera may be possible. While power switch SW₁ is closed, a low level signal applied through line 5 to holding timer 24 puts timer 24 into a reset condition where timer 24 can not operate. At this time, the output transmitted from timer 24 to AND gates G₁ and G₂ is a high level. Accordingly, AND gates G₁ and G₂ produce high level outputs and in turn the output of OR gate G₃ is a high level. Diode D₂₁, capacitor C₁₂, resistors and transistors Q₂₁ and Q₂₄ constitute a reset circuit for power supply holding. The high level output of OR gate G₃ places transistor Q₂₁ in its ON state which turns transistor Q₂₄ into OFF state. Thus, the power supply holding operation in transistors Q₁₀ and Q₂₀ is not affected. When power switch SW₁ is opened, timer 24 receives a high level signal through reverse current blocking diode D₂₀ and pull-up resistor R₂₀ and starts timing operation by counting clock pulses forwarded from digital arithmetic circuit 20. Te signal applied to gates G₁ and G₂ from timer 24 still remains high and the time required to change from a high level to a low level is longer in the signal applied to gate G₁ than in the signal applied to gate G₂. Accordingly, as the signal applied to gate G₁ from timer 24 goes low, the output of OR gate G₃ goes low and transistor Q₂₁ is turned off. The accumulated charge of capacitor C₁₂ in the reset circuit keeps transistor Q₂₄ in its ON state for a predetermined time. Transistor Q₂₄ in its ON state turns off transistor Q₂₀ of the power supply holding circuit so that the positive feedback is cleared. Thus, transistor Q₁₀ is securely turned off and the power supply is terminated. While the shutter is working after the camera has been actuated by the closure of switch SW₂, an exposure operation signal of low level is transmitted from digital arithmetic circuit 20 through line 5 to timer 24. As long as this signal is transmitted, timer 24 does not start the time counting operation even if switch SW₁ is opened. And also, since the ON condition of transistor Q₁₀ is held through diode D₂₀ and line 5 without regard to the situation of switch SW₁ and transistor Q₂₀, the power supply can not terminate while the shutter is being opened and the shutter control can not be disturbed. When the exposure is completed by the closure of the shutter, the exposure operation signal on line 5 becomes a high level and then timer 24 starts its operation. The power supply terminates after a corresponding time has elapsed. As a result of the battery voltage drop, when the relation of V_(A12) <V_(ES) <V₁₁ appears, the output of comparator A₁₂ only is in a low level. Thus the output of gate G₁ is a low level. The power supply holding time is controlled by a shorter time signal transmitted to the gates from timer 24, that is the output of gate G₂, in the same manner as the above. A user can learn from the shorter power supply holding time that battery consumption is developing.

As the battery voltage further goes down and falls into the relation of V_(A11) <V_(ES) <V_(A10), comparator A₁₂ and A₁₁ produce a low level output and comparator A₁₀ a high level output. When power switch SW₁ is closed in this situation, the outputs of gates G₁ and G₂ become low by receiving the outputs of comparators A₁₁ and A₁₂. Since transistor Q₂₁ is in its OFF state and transistor Q₂₄ in its ON state, transistor Q₂₀ of the power supply holding circuit is turned off. Accordingly, the positive feedback does not function and thus the holding time becomes zero. That is, as power switch SW₁ is opened, the display immediately goes out. From this fact, the user can find that the battery will be used up to the extent of an unusable condition soon.

Where the battery comsumption has developed still further so that the battery voltage falls below a lower limit value capable of ensuring the proper exposure control, that is V_(A10) <V_(ES), upon the closure of power switch SW₁ the positive feedback does not function because all the outputs of comparators A₁₀, A₁₁ and A₁₂ are low, though transistor Q₁₀ is turned on. Accordingly, the power supply holding time is zero. Transistor Q₁₃ is turned off because comparator A₁₀ produces a low level output, and thus the display remains unlighted. Even if release switch SW₂ is erroneously closed, the photographing sequence will not start because the release operation is locked, that is, transistor Q₁₄ can not be turned on. The user will notice that the battery has been used up to an unusable condition.

FIG. 6 shows the relation between battery voltage V and holding time t after power switch SW₁ is opened in the circuit of FIG. 5, where V₂₂ is a battery voltage at V_(A12) <V_(ES) <V_(A11), V₂₁ a battery voltage at V_(A11) <V_(ES) <V_(A10) and V₂₀ a battery voltage at V_(A10) <V_(ES). The line of FIG. 6 shows that the holding time is set as t₁₀ in the battery voltage range of V>V₂₂, t₁₁ shorter than t₁₀ in the range of V₂₁ <V<V₂₂ and zero in the range of V₂₀ <V<V₂₁. And in the battery voltage range of V<V₂₀, shutter release locking is in effect.

In the power supply control circuit according to the invention, the waste of a battery can be relieved because the power supply holding time is made shorter as battery consumption develops.

The present invention is not limited to the embodiment shown, but can be modified without deviating from the scope of the present invention. 

I claim:
 1. A power supply control circuit for controlling supply of power from a battery to a battery-operated circuit, the control circuit comprising:manually operable power switch means; means responsive to manual operation of said power switch means for supplying power from said battery to said battery-operated circuit as long as said power switch means is manually operated; a time-measuring circuit having means for starting time-measuring upon termination of manual operation of said switch means and producing a time-measuring signal for a predetermined time period that begins with the starting of time-measuring; a holding circuit having means for supplying power from said battery to said battery-operated circuit while said time-measuring signal is being produced; and a detection circuit for detecting the battery voltage and reducing said predetermined time period as the battery voltage goes down.
 2. The power supply control circuit as defined in claim 1, wherein said detection circuit has means that produces a first detection signal when the battery voltage is higher than a predetermined value and a second detection signal when the battery voltage is lower than said predetermined value.
 3. The power supply control circuit as defined in claim 2, wherein said time-measuring circuit has means operative in response to said first detection signal to set said predetermined time period to a first time period and in response to said second detection signal to set said predetermined time period to a second time period markedly shorter than said first time period.
 4. The power supply control circuit as defined in claim 3, wherein said second time period is substantially zero.
 5. The power supply control circuit as defined in claim 2, wherein said means that produces said first detection signal continuously changes said first detection signal in response to the fall of the battery voltage, and wherein said time-measuring circuit has means that continuously reduces its said predetermined time period as said first detection signal changes.
 6. The power supply control circuit as defined in claim 1, wherein said detection circuit has means that prevents the supply of power from said battery to said battery-operated circuit when the battery voltage is below a lower limit value irrespective of the manual operation of said switch means, the lower limit value being a voltage at which said battery-operated circuit cannot work in a normal manner.
 7. The power supply control circuit as defined in claim 6, wherein said battery-operated circuit includes display means which operates as long as power is supplied. 